Circle Calculator
Calculate circle radius, diameter, circumference, and area from any one known value. Instant results with adjustable precision.
Updated
What is the Circle Calculator?
A circle calculator finds every primary measurement of a circle—radius, diameter, circumference, and area—from any single known value. Enter radius, diameter, circumference, or area and the remaining three quantities update instantly.
All circle properties are linked by the constant π (pi ≈ 3.14159…):
- Diameter is twice the radius: (d = 2r)
- Circumference is the distance around the circle: (C = 2\pi r = \pi d)
- Area is the space inside the circle: (A = \pi r^{2})
Because the relationships are exact, knowing one value is enough to recover the others. Units stay consistent: if you enter radius in centimetres, diameter and circumference are in centimetres and area is in square centimetres.
This tool is useful for geometry homework, engineering sketches, craft projects, landscaping, and any situation where you need quick, accurate circle dimensions without rearranging formulas by hand. Adjust decimal precision from 2 to 10 places to match the accuracy your work requires.
How it works
Choose the known quantity
Select Radius, Diameter, Circumference, or Area with the mode control.Enter a non-negative number
Type the value you know. Results update live as you type. Negative or non-numeric input is rejected with a clear message.Review the four results
The calculator derives the other three measurements and displays all four together. The value you entered is shown alongside the derived ones so you can verify consistency.Set decimal places
Choose 2, 4, 6, 8, or 10 decimal places. Formatting trims unnecessary trailing zeros while keeping the requested precision available for copy.Copy or export
Use the summary block to copy radius, diameter, circumference, area, and the formulas used for the active mode.
Formulas used
| Given | Radius | Diameter | Circumference | Area |
|---|---|---|---|---|
| Radius (r) | (r) | (2r) | (2\pi r) | (\pi r^{2}) |
| Diameter (d) | (d/2) | (d) | (\pi d) | (\pi (d/2)^{2}) |
| Circumference (C) | (C/(2\pi)) | (C/\pi) | (C) | (C^{2}/(4\pi)) |
| Area (A) | (\sqrt{A/\pi}) | (2\sqrt{A/\pi}) | (2\sqrt{\pi A}) | (A) |
Calculations use the full JavaScript Math.PI constant. Zero input correctly yields zero for every property. Extremely large or tiny magnitudes switch to scientific notation so results remain readable.
Examples
From radius
Given radius 5, compute diameter, circumference, and area.
From diameter
A 12-inch pizza diameter yields radius, circumference, and area.
From circumference
Fence length around a circular garden equals circumference 25.13 m.
From area
Given area 100, recover radius, diameter, and circumference.
Frequently asked questions
How do I calculate the area of a circle?
How do I calculate the area of a circle?
Use the formula (A = \pi r^{2}). Enter the radius (or diameter, circumference, or area) in the calculator and the area is computed automatically. If you only know the diameter, the tool first finds (r = d/2) then applies (\pi r^{2}).
How do I find the circumference of a circle?
How do I find the circumference of a circle?
Circumference is the distance around the circle: (C = 2\pi r) or equivalently (C = \pi d). Enter any one known value and the calculator returns the circumference along with the other properties.
Can I find the radius if I only know the area?
Can I find the radius if I only know the area?
Yes. The radius is (r = \sqrt{A / \pi}). Select Area as the input mode, enter the area, and the calculator returns radius, diameter, and circumference.
What is the difference between radius and diameter?
What is the difference between radius and diameter?
The radius is the distance from the centre of the circle to any point on the edge. The diameter is the distance across the circle through the centre and is exactly twice the radius: (d = 2r).
Why is π used in circle formulas?
Why is π used in circle formulas?
Pi (π) is the constant ratio of a circle’s circumference to its diameter. That ratio is the same for every circle, approximately 3.141592653589793. Because of this fixed relationship, circumference, area, radius, and diameter are all interconvertible.
Do units matter in the calculator?
Do units matter in the calculator?
The calculator works with pure numbers. Results share the same length unit as your input (for radius, diameter, and circumference) and the corresponding square unit for area. Enter values in the unit you care about—cm, m, in, ft—and interpret the outputs in matching units.
What happens if I enter zero or a negative number?
What happens if I enter zero or a negative number?
Zero is valid and produces zero for every property. Negative values are rejected because radius, diameter, circumference, and area cannot be negative.
How precise are the results?
How precise are the results?
Results use the full floating-point value of π available in the browser. You can display from 2 to 10 decimal places. For most practical work 4–6 decimals is sufficient; higher precision is available when needed.
Related tools
More utilities you might find handy.
Cube Root Calculator
Calculate cube root of any number
Decimal & Base Converter
Instantly convert numbers between decimal, binary, hexadecimal, octal, and any custom base from 2 to 36.
Factorial Calculator
Calculate exact factorials for non-negative integers with BigInt precision, digit count, and step-by-step breakdowns.